The present invention relates to a prosthetic patello-femoral joint assembly and, more particularly, to a mobile bearing patellar prosthesis with orbital translation such that the articulation component moves in an infinite number of directions with respect to the baseplate.
In the United States alone, over 200,000 knee replacements are performed each year. Degenerative arthritis, or the gradual degeneration of the knee joint, is the most common reason for these replacements. In this form or arthritis, cartilage and synovium surrounding the knee wear down so underlying bones grind directly on each other.
In knee arthroplasty, portions of the natural knee joint are replaced with prosthetic components. These components include a tibial component, a femoral component, and a patellar component. The femoral component generally includes a pair of spaced condyles that articulate with the tibial component. These condyles form a trochlear groove in which the articulating surface of the patellar component moves. The components are made of materials that exhibit a low coefficient of friction when they articulate against one another.
When the articulating ends of both the femur and tibia are replaced, the procedure is referred to as total knee replacement or TKR. Much effort has been devoted to performing TKR that restores normal, pain-free functions of the knee for the lifetime of the prosthetic components.
Unfortunately, patients can experience problems with the prosthetic knee after a total knee replacement surgery. If a problem occurs, a patient may need a revision surgery wherein some or all of the prosthetic components are replaced. Historically, problems associated with the patellar prosthesis are responsible for as many as 50% of all knee implant revisions. More particularly, complications with the patello-femoral joint or patello-femoral dysfunction are the primary cause of failure in TKR.
One option in a TKR or revision surgery is to implant a prosthetic patellar component. The patellar component has a metallic back or baseplate that is permanently fixed to the patellar bone. Metal baseplates were introduced to provide a more even stress distribution on the natural patella and provide the option for either cement or cementless fixation. An articulation or bearing component is permanently connected to the baseplate to form the prosthetic patellar component. The articulation component is formed from metal or a polymer, such as ultra-high molecular weight polyethylene (UHMWPE).
Typically, the articulation component can move relative to the baseplate. This movement is extremely important to the success and proper function of the prosthetic patella. As a normal knee proceeds through a full range of flexion, the patella actually moves in several directions as it tracks along the trochlear groove. Even under normal motion, for example, the patella can move both medially and laterally, with the actual movement of the patella having a much more complicated pathway. Not surprisingly then, much effort has been devoted to designing a prosthetic patella that emulates the movement of a natural patella.
U.S. Pat. No. 5,702,465 entitled xe2x80x9cPatella Prosthesis Having Rotational and Translational Freedomxe2x80x9d to Burkinshaw teaches a two-piece prosthetic patella in which the articulation component moves in two different directions with respect to the baseplate. Specifically, the articulation component can rotate about the baseplate and move vertically in a longitudinal channel in the baseplate. U.S. Pat. No. 5,609,644 entitled xe2x80x9cProsthetic Patello Femoral Joint Assemblyxe2x80x9d to Ashby et al. teaches a two-piece prosthetic patella in which the articulation component moves in three different directions with respect to the baseplate. Specifically, the articulation component can rotate about the baseplate and move vertically in a longitudinal channel in a manner somewhat similar to U.S. Pat. No. 5,702,465. The design in Ashby, though, allows for a small amount of medial-lateral shift while the articulation component tracks along the longitudinal groove.
Despite current advances in the design of the connection between the articulation component and the baseplate, prosthetic patellae still do not fully emulate the natural movement of the patella. Specifically, these prior designs limit the movement of the articulation component with respect to the baseplate. Typically, these movements occur in straight, axial directions, such as movement down a longitudinal channel or movement in a purely medial-lateral direction while in this channel.
It, therefore, would be advantageous to provide an implantable patellar prosthesis that could closely emulate the natural movement of a patella. This prosthesis would have an articulation component that could move in an infinite number of directions with respect to the baseplate.
The present invention relates to a prosthetic patello-femoral joint assembly used to replace a portion of the natural knee and, more particularly, to a mobile bearing patellar prosthesis with orbital translation such that the articulation component moves in an infinite number of directions with respect to the baseplate. The patellar prosthesis comprises two basic components: A baseplate and an articulation component.
Each baseplate has a fixation surface and a bearing surface. The fixation surface is adapted to engage patellar bone and includes a plurality of pegs that extend outwardly from the surface to penetrate bone. The bearing surface connects to the articulation component as described herein.
Each articulation component has an articulation surface and a bearing surface. The articulation surface has a smooth contour that is adapted to articulate with a femoral component, such as a natural femur or femoral prosthesis at the patello-femoral joint. This surface may have various shapes known to those skilled in the art, such as a hyperbolic paraboloid, saddle-shape, or dome-like configuration. The bearing surface of the articulation component is adapted to engage the bearing surface of the baseplate. These surfaces are configured to lock together and then slideably contact or articulate with each other.
An attachment mechanism couples the baseplate to the articulation component so the bearing surfaces are adjacent each other. The attachment mechanism can have a variety of configurations to enable the articulation component to engage and articulate with the baseplate. In one embodiment, this mechanism includes an attachment member that protrudes from the bearing surface of the baseplate. The attachment member has a cylindrical shaft with an enlarged head. This head has a circular body portion with two wings that extend from opposite ends of the circular body. The articulation component includes a recess or keyway shaped to receive the enlarged head of the attachment member. This recess extends into the body of the articulation component and includes an undercut.
In operation, the head of the attachment member is inserted into the recess in the articulation component. The articulation component is then rotated so the wings of the head extend along the undercut. In this position, the articulation component and baseplate are engaged and locked together since the wings are captured inside the undercut.
As one important advantage of the present invention, the articulation component can move in an infinite number of directions with respect to the baseplate. Specifically, the diameter of the head of the attachment member (as measured from the ends of the wings) is less than the diameter of the bearing surface of the articulation component but greater than an inner diameter of the undercut. In other words, while the articulation component remains captured to the baseplate, the wings of the attachment member do not completely extend the full distance into the undercut. This difference in distance enables the articulation component to move while engaged and captured to the baseplate. Most importantly, the movement of the articulation component with respect to the baseplate is not limited to one or two axial directions. Instead, the articulation component can freely move in the X-Y plane in an infinite number of directions with respect to the baseplate. This movement can occur in a circular pathway along the undercuts. As such, the articulation component has xe2x80x9corbital translationxe2x80x9d with respect to the baseplate.
As another advantage of the present invention, the articulation component is removeably connectable to the baseplate. In other words, even after the baseplate becomes permanently connected to the patellar bone, an articulation component can be readily attached or detached from the baseplate. During a revision surgery then, healthy bone stock of the natural patella will not be damaged or removed since the baseplate can be left attached to the patella.
As yet another advantage, the attachment mechanism of the patellar prosthesis can be utilized with various designs of articulation components and baseplates. In one embodiment, for example, the patellar prosthesis comprises only two separate or individual components: A base component and an articulation component. No other components are required to form and connect the patellar prosthesis with orbital translation. Both the articulation component and the baseplate are formed as a single unit or piece. In other words, these components are not formed from multiple pieces assembled together, but from a unitary, integral unit or piece. Further, these two components include an attachment mechanism that is integrally formed to either or both components. As such, no separate attachment mechanism is required to couple the baseplate and articulation component. In another embodiment, a third or intermediate component is used to connect the articulation component and baseplate. This third component forms the bearing surface of the articulation component and houses a portion of the attachment mechanism needed to engage with the baseplate.
Other objects and advantages of the present invention will be apparent from the following descriptions of a preferred embodiment with reference to the drawings.